. >> reporter: day four, president obama comes to boston and speaks of an interfaith service telling boston the country stands with it. >> the world will return to this great american city to run harder than ever and to cheer even loird for t-- louder for t 111 118th boston marathon. bet on it. >> reporter: later in that day, a break in the as i. the fbi released surveillance photo of this tease men walking with backpack. at 10, 48 p.m. gunshots are heard on the campus of m.i.t. in cambridge, massachusetts. reports come in that campus officer sean collier was killed. shortly after the two men carjack a black mercedes and a chase ensues culminating with a shootout in watertown where sun suspect revealed to be tamerlan tsarnaev was killed. day five, witnesses describe the mayhem. >> we saw the explosion. we must have heard about 60 gunshots. >> reporter: 2:40 a.m., a robocall sent to watertown residents. >> there was an active incident in watertown right now. chief defoe is advising all watertown resident to remain in their homes. >> reporter: 4:22 a.m., a suspect on the run. >> we bel